Method of making a closed porosity surface coating on a low density preform

ABSTRACT

A method of making a low density part with a closed porosity surface coating is formed by applying, to a porous powder preform, a coating of powder finer than that of the preform which is sinterable to a near full density below the melting temperature of the powder; and heating the coated preform to sinter the coating to form a near full density, gas impermeable, closed porosity surface coating on the preform.

FIELD OF INVENTION

[0001] This invention relates to manufacturing near net shape powder metal parts and more particularly to a method of manufacturing a near net shape powder metal part with a closed porosity surface using a powder metal coating.

BACKGROUND OF INVENTION

[0002] Manufacturing of parts by consolidating metal powders offers the potential of greatly reduced manufacturing costs when compared to more traditional manufacturing such as cast or wrought parts. To date, however, this potential has been realized only with part shapes and powder that are amenable to die pressing or metal injection molding followed by a sintering operation. To a large extent, powder metallurgy techniques have not been amenable to economically produce parts that are large, have complex shapes and have a high density, e.g., near 100%. This is still true, despite the fact that many processes have been developed over the last twenty years directed to obtaining near 100% dense parts by compacting metal powders. These processes include Hot Isostatic Pressing (HIP), Rapid Omnidirectional Compaction (ROC), Pneumatic Isostatic Forging (PIF), Rapid Isostatic Pressing (RIP) and the Ceracon process.

[0003] Each of these processes rely on hydrostatic or near hydrostatic compression forces being applied to the powder while maintaining the powder at a high temperature. Due to the porosity of the powder, it is necessary to place the mass in a sealed can in order to transmit the compressive forces to the powder.

[0004] U.S. Pat. Nos. 3,700,435, 5,094,810 and 5,217,227 teach forming an article by placing the powder in a mold, placing the mold in a secondary media, such as sand, and then sealing the mold and sand in a can and finally hot degassing the interior of the can. The temperature is then elevated and pressure increased to compact the sand and thus compact the metal powder. However, this process is time consuming and expensive due to the many processing steps required.

[0005] U.S. Pat. No. 3,982,934 teaches electroplating metal cans over a disposable preform. The preform is removed and the can is filled with metal powder and then sealed. Pressure is increased to compact and consolidate the metal powder. However, this method suffers from hot tearing of the can and subsequent loss of compactability due to the loss of the can's integrity as well as reaction between the powder and the metal can.

[0006] U.S. Pat. No. 3,622,313 teaches using a glass or vitreous container instead of a can for applying rapid omnidirectional compaction (ROC). However, the glass, like the can above, tends to react with the powder. Moreover, the part must still be machined to remove the glass.

[0007] Others have attempted to produce a can which is the same shape as the finished part. See U.S. Pat. Nos. 5,561,834 and 4,487,096. A part is stamped or sintered to obtain the desired shape. The part is thereafter oxidized to provide a gas impermeable layer which allows the part to be compacted. The oxide layer must then be machined off.

[0008] U.S. Pat. No. 5,640,667 teaches an exact shape can using insitu lasering of the metal powder one layer at a time in layers which are only thousandths of inches thick. Once the entire part is lased, producing a shaped can, the part undergoes hot isostatic pressing (HIP) to compact the metal powder. This method, however, is very time consuming due to the many thin layers that are required, e.g. it may take two days or more to produce one can. The part is then compacted by HIP. This method is not an effective method for mass manufacturing of powder metal near net shape parts.

[0009] Finally, U.S. Pat. No. 5,816,090 teaches that the need for a can may be circumvented by applying gas pressure to the preform very rapidly so that the gas does not have time to penetrate the preform thereby compacting the preform. However, this method requires a preform with a density of near 90%. Typical sinter densities, however, are approximately 80%, which is not sufficient for this method to work.

[0010] Most of these processes start with loose powder that is poured into a can which is evacuated and sealed and subsequently compacted. The can is designed so that after compaction, the powder in the can loosely approximates the final shape of the desired part. When the part shape is very complex or large, however, can fabrication is very expensive. To reduce the cost of the cans, the quality of the shape is sacrificed by providing an excessively large part envelope. This results in excessive and unnecessary use of expensive powder to fill the can and unnecessary machining to bring the part to its final geometry after the consolidation process. Moreover, because the can is usually a different material than the powder, and because it is a continuous solid rather than a powder, it reacts differently than the powder to the applied pressures. This fact severely complicates the designer's job in predicting the can shape necessary for a particular part.

[0011] Given the problems of can design, the need to use oversized cans, the cost in fabricating the cans, the sacrificed shape to accommodate cost and the machining requirements to remove the can as well as the need to machine the part to obtain the final shape, powder metallurgy is not effectively being used to produce large, complex, near net shape parts.

BRIEF SUMMARY OF THE INVENTION

[0012] It is therefore an object of this invention to provide a method of making a metal powder near net shape part with a closed porosity surface coating.

[0013] It is a further object of this invention to provide such a method which does not require a can to consolidate the powder.

[0014] It is a further object of this invention to provide such a method in which the closed porosity surface coating acts as a can for consolidating the powder.

[0015] It is a further object of this invention to provide such a method in which the coating is a near net shape.

[0016] It is a further object of this invention to provide such a method in which the coating is integral with the near net shape part.

[0017] It is a further object of this invention to provide such a method which does not require machining the coating off of the near net shape part.

[0018] It is a further object of this invention to provide such a method in which the final part requires only finish machining.

[0019] It is a further object of this invention to provide such a method which produces a near net shape powder metal part with near full density.

[0020] It is a further object of this invention to provide such a method which produces a gas free near net shape powder metal part.

[0021] It is a further object of this invention to provide such a method which produces a near net shape part with a closed porosity surface coating of a material the same as or different than the preform.

[0022] It is a further object of this invention to provide such a method which is cost effective to implement.

[0023] The invention results from the realization that a truly near net shape part may be accomplished by applying to a powder preform a coating of fine powder, finer than that of the preform, and heating the powder coated preform to sinter the coating to form a near full density, gas impermeable, closed porosity surface coating on the preform. The part may be utilized as is, or the part may be subjected to increased pressure to further compact the preform to achieve a near net shape part of predetermined density.

[0024] The invention features a method of making a low density part with a closed porosity surface coating by applying, to a porous powder preform, a coating of powder finer than that of the preform which is sinterable to a near full density below the melting temperature of the powder, and heating the coated preform to sinter the coating to form a near full density, gas impermeable, closed porosity surface coating on the preform.

[0025] In a preferred embodiment an increased pressure may be applied to the closed porosity surface coating for transmitting the force of the pressure through the coating to consolidate the porous powder preform to a predetermined density. The increased pressure may include gas pressure. The preform may be consolidated to near full density. The step of applying may include adding the metal powder to a carrier and applying the carrier to the preform. The carrier may include a binder for aiding the powder in adhering to the preform. Applying may include brushing the powder onto the preform, dipping the preform into the powder or spraying the powder onto the preform. Spraying may include melting the powder and spraying the molten droplets onto the preform. The coated part may be placed in a vacuum furnace and the pressure reduced to remove any gas from the porous preform prior to heating. The powder may spherical. The coating of powder may be the same material as the preform.

[0026] The invention also features a method of making a high density part from a low density part by applying, to a porous powder preform, a coating of powder finer than that of the preform which is sinterable to a near full density below the melting temperature of the powder coating, heating the powder coated preform to sinter the coating to form a near full density, gas impermeable, closed porosity surface coating on the preform, and increasing pressure to the closed porosity surface coating for transmitting the force of the pressure through the coating to consolidate the porous powder preform to a predetermined density.

[0027] In a preferred embodiment the increased pressure may include gas pressure. The preform may be consolidated to near full density. The step of applying may include adding the metal powder to a carrier and applying the carrier to the preform. The carrier may include a binder for aiding the powder in adhering to the preform. Applying may include brushing the powder onto the preform, dipping the preform into the powder or spraying the powder onto the preform. Spraying may include melting the powder and spraying the molten droplets onto the preform. The coated part may be placed in a vacuum furnace and the pressure reduced to remove any gas from the porous preform prior to heating. The powder may be spherical. The coating of powder may the same material as the preform.

[0028] The invention also features a method of making a metallic part with a closed porosity surface coating by applying, to a metallic part, a coating of powder sinterable to a near full density below the melting temperature of the powder, and heating the coated part to sinter the powder coating to form a near full density, gas impermeable, closed porosity surface coating.

[0029] In a preferred embodiment an increased pressure may be applied to the closed porosity surface coating for transmitting the force of the pressure through the coating to consolidate the porous powder preform to a predetermined density. The increased pressure may include gas pressure. The preform may be consolidated to near full density. The step of applying may include adding the metal powder to a carrier and applying the carrier to the preform. The carrier may include a binder for aiding the powder in adhering to the preform. Applying may include brushing the powder onto the preform, dipping the preform into the powder or spraying the powder onto the preform. Spraying may include melting the powder and spraying the molten droplets onto the preform. The coated part may be placed in a vacuum furnace and the pressure reduced to remove any gas from the porous preform prior to heating. The powder may spherical. The coating of powder may the same material as the preform.

[0030] The invention also features a low density powder metal near net shape part with a closed porosity coating produced by applying, to a porous powder preform, a coating of powder finer than that of the preform which is sinterable to a near full density below the melting temperature of the powder, and heating the coated preform to sinter the coating to form a near full density, gas impermeable, closed porosity surface coating on the preform.

[0031] The invention also features a near net shape part having a porous metal powder preform, and a fine metal powder coating disposed on said preform, the metal powder coating sintered below the melting temperature of the powder coating to form a full density, gas impermeable closed porosity coating on the preform.

[0032] The invention also features a method of making a near net shape full density coating by applying a powder coating to a powder metal preform, the powder coating sinterable to full density, and heating the coated preform to sinter the coating to form a closed porosity, gas impermeable near net shape coating.

[0033] In a preferred embodiment the powder of the coating may be finer than that of the preform.

[0034] The invention also features a method of making a low density part with a gas impermeable coating by applying, to a porous powder preform, a coating of powder finer than that of the preform and having a lower melting point than the preform and heating the powder coated preform to melt the coating such that the coating reacts with the preform to form a near full density, gas impermeable, closed porosity surface coating on the preform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:

[0036]FIG. 1 is a block diagram generally showing the method of the present invention;

[0037]FIG. 2 is a more detailed block diagram of the method of FIG. 1;

[0038]FIG. 3 is a block diagram, similar to FIG. 2, in which the powder is applied to the inside of a mold surface prior to being applied to the preform;

[0039]FIG. 4 is an example of a near net shape porous metal powder preform prior to application of the fine metal powder coating;

[0040]FIG. 5 is a micrograph of the preform depicted in FIG. 4 demonstrating the porosity of the preform;

[0041]FIG. 6 shows the preform of FIG. 4 after a fine metal powder, different than that of the preform, has been sintered to form a fully dense, closed porosity surface coating according to the method of the present invention;

[0042]FIG. 7 is a micrograph of the preform depicted in FIG. 6 demonstrating the fully dense, closed porosity surface coating;

[0043]FIG. 8 is micrograph, similar to FIG. 7, in which the fully dense, closed porosity coating is the same material as the preform; and

[0044]FIG. 9 is an example of the metal powder applied to a mold surface according to the diagram of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0045] As discussed in the Background of the Invention above, powder metallurgy has not historically been economically feasible for producing large, complex near net shape parts. Economic success has been hindered due to the need for expensive prior art cans and post machining operations of not only the final geometry of the part, but also machining of the prior art can off the final part.

[0046] The method of the present invention, however, solves these problems by producing near net shape parts using porous, low density powder metal preforms and forming on the preform a hermetic or closed porosity surface coating or skin. Generally, a separate can is required to translate gas pressure to the powder preform, due to its porosity, in order to consolidate and compact the preform to increase its density. Without a can, the gas pressure passes right through the preform and no consolidation occurs. The expense of the prior art can is reduced by lessening the detail of the part thus requiring additional machining other than that required to remove the can in order to obtain the final shape. Where the material is, for example, titanium, the wasted material in the form of machining scrap can be very expensive.

[0047] By forming a closed porosity surface coating on the preform, no expensive can is needed. Instead, the surface acts as the can. Moreover, this “can” becomes an integral part of the preform and need not be machined off. Thus, a near net shape preform may be used and a near net shape coating is produced, as the coating exactly conforms to the shape of the preform, minimizing machining costs and unnecessary waste of expensive material.

[0048] According to the method of the present invention, a metal powder, step 10, FIG. 1, is applied to a low density preform, step 12, to coat the preform. The preform may be manufactured by cold isostatic compaction, metal injection molding, or other techniques well known in the art. However, the preform may also be manufactured as taught in a co-pending application entitled METHOD OF MAKING HIGH DENSITY SINTERED METAL ARTICLES, incorporated herein by this reference. The coated preform is then heated to a temperature below the melting point of the powder, typically 0.7-0.9 of the melting point, to sinter the powder to form a full or near full density, closed porosity surface coating which is gas impermeable. Small powder particles will sinter to a much higher density than larger particles. Additionally, very fine particles can be sintered to full density at temperatures which will not sinter larger particles. Depending on the desired application, the low density sintered part may be machine finished for use, or the sintered preform may be consolidated, step 16, shown in phantom. Increased pressure is applied to the gas impermeable, closed porosity coating which transmits the force of the pressure, through the coating, to consolidate and compact the preform to the desired density, e.g. near full density. This compaction may be accomplished by cold isostatic pressing, i.e., simply increasing gas pressure applied to the surface. Thus, there is no need to heat the part. However, other compaction techniques which require heat may also be used, e.g., HIP, RIP, ROC, and Ceracon consolidation methods.

[0049] Thus, the present invention enables powder metal near net shape parts with near full density to be produced without the need for expensive can production, wasted material or expensive machining.

[0050] Fine metal powder 18, FIG. 2 may typically be in the range of 1-44 microns. Nanopowders, powders less than 1 micron in diameter, will fuse more rapidly and at lower temperatures than the 1-44 micron fine powders. In any case, the fine metal powder must be at least smaller than the powder of the preform. While spherical powders are preferred, this is not a limitation as non-spherical powders will also work. In order to improve the application of the powder to the preform, the powder may be combined with a carrier, step 20, such as alcohol, water, or other volatile organic, however this is not a necessary limitation of the invention, as the powder may be applied directly to the preform. Whether or not a carrier is used, the metal powder is applied to the preform, for example with a brush, sprayer, or by dipping the preform in the powder or carrier combination, dipping the part in a fluidized bed of powder or even melting the powder and spraying onto the preform as molten droplets, e.g. plasma spraying. If a carrier is used, the carrier must be allowed to evaporate.

[0051] Once the metal powder is applied such that a visible and continuous coating is obtained, the coated preform is heated below the melting temperature of the powder, step 24, in a furnace.

[0052] Once the metal powder has sintered to full or near full density to provide a closed porosity surface coating, the sintered, coated preform is removed from the furnace.

[0053] In certain applications, the part's intended use may be for high temperature applications. If there is gas within the preform, the high temperatures will cause the gas to expand, affecting the strength and integrity of the part, causing the part to eventually fail. Thus, in high temperature applications the coated preform may be placed in a vacuum furnace and the pressure reduced. Because the coating is still a powder at this point, the furnace must be evacuated sufficiently slow so that the powder will not be dislodged from the preform by the escaping gas.

[0054] No matter which technique is used, however, the result is a low density powder metal, near net shape part with a closed porosity surface coating, step 26. Again, depending on the application, the part produced may be machine finished and ready for use.

[0055] However, where higher density is necessary the sinter coated preform, step 24, may be consolidated by increasing the environmental pressure such as by cold isostatic pressing or by standard hot consolidation such as HIP, RIP, ROC and Ceracon techniques, step 30. By using cold isostatic pressing, costs are reduced since no heating is required. Because the surface coating has a closed porosity, cold isostatic pressing is preferred because the increased gas pressure does not pass through the coating, but acts on the coating which in turn transmits the force of the gas pressure to the powder preform to compact and consolidate the powder of the preform to produce a powder metal, near net shape, near full density part, step 32. However, this is not a necessary limitation of the invention as the density required may be a predetermined density somewhere between the preform density and full density. Thus, the coating acts as a can which becomes integral with the preform. In the case where the coating is to be a material of a lower melting point than the preform, the preform can even be dipped in a molten bath of the material. Pressure and the vacuum infiltration techniques could be easily used to adjust the depth of penetration of the coating.

[0056] Another feature of the present invention is that the fine metal powder may be sintered in the same step as the preform yet still eliminates the need for a prior art can. The fine metal powder may be combined with another medium to form a slurry, step 34, FIG. 3, and poured into the mold, step 36, for example a ceramic disposable shell mold or any standard mold. However, a ceramic disposable shell mold is used to produce large, complex parts. The slurry may be a combination of the metal powder and, for example, a carrier for dispersing the powder and a wetting agent to allow the slurry to adhere to the mold surface. While slurries can be made of many different types of metal powder it is important that the powder not rapidly settle out of the slurry, and that the slurry fully wet the powder. To this end the smaller the powder used the better. With the interior of the mold sufficiently coated, the mold is filled with preform powder, step 38.

[0057] After coating the mold and filling with preform powder, the mold is heated to sinter the coating and preform powder, step 40. Because the powder coating consists of fine particles, the coating will sinter to form a closed porosity surface first. As the temperature increases, the preform powder will sinter. Once the mold is removed, step 42, the low density near net shape part with closed porosity coating may be machined for use, step 44, or the part may undergo consolidation, step 46, to produce a near net shape, near full, or predetermined density, step 46, as discussed above.

[0058] Alternatively, once the slurry is poured into the mold, step 36, the coated mold may be heated, step 37, to sinter the powder coating to produce a near net shape closed porosity coating inside the mold. The mold with sintered coating may then be filled with perform powder, step 39, and the mold heated a second time to sinter the preform powder to the closed porosity coating, step 41. After the second heating, the mold is removed, step 42, and the part machined for use or consolidated as discussed above.

[0059] To demonstrate the feasibility of the present invention, a 1.5 kg porous powder preform 52, FIG. 4, was sintered using less than 35 mesh, typically having a particle size less than 500 microns, Ti 6Al-4V powder (six percent aluminum and four percent vanadium). The porosity of the powder preform is demonstrated by its dull appearance and lack of metallic reflectivity. Preform 52, FIG. 5, had a density of approximately 80%. As the micrograph demonstrates, surface 54 of the preform 52 is very porous and no consolidation could take place as the gas would easily flow through preform 52.

[0060] Fine aluminum powder, 2-5 micron, was suspended in an ethanol solution and painted onto preform 52 using a paint brush. The ethanol carrier rapidly wicked into the interior of porous preform 52 leaving the aluminum powder loosely caked to the preform. The carrier was allowed to evaporate at room temperature for twenty four hours.

[0061] Preform 52 was placed in a vacuum furnace and slowly evacuated to 5×10⁻⁵ torr to avoid dislodging the powder from the preform. The temperature was increased to 1200°C. at 10°C./s and held thereafter for 120 minutes to sinter the aluminum powder to the Ti-6Al-4V preform by transient liquid phase sintering.

[0062] The closed porosity surface 56, FIG. 6, of sintered preform 58 is demonstrated by the metallic reflectivity due to the full density fused coating.

[0063] The result is more clearly demonstrated by closed porosity surface 56, FIG. 7. As can be seen from the micrograph, the sintered preform 58 includes porous preform 52 with a closed porosity, fully dense coating 56.

[0064] In a similar manner, a second less than 35 mesh, typically having a particle size less than 500 microns, Ti 6Al-4V powder preform was coated with less than 44 micron Ti-6Al-4V using ethyl alcohol as a carrier. The coated preform was vacuum sintered at 1500°C., below the melting point of Ti-6Al-4V, for 120 minutes. As above, the sintered preform emerged with a highly reflective surface indicating the fully dense closed porosity coating. The closed porosity surface of sintered coated preform 58′, FIG. 8, is demonstrated by continuous surface 56′ covering porous preform 52′. Accordingly, one metal may be coated onto a preform of another.

[0065] Thus, as demonstrated above, the method of the present invention produces a closed porosity surface coating on a porous powder preform. Accordingly, the coating need not be the same material as the preform.

[0066] Further, the preform need not be a porous powder preform as described above. The preform may be an already near fully dense preform such as a cast steel sea water valve. The valve can then be coated with titanium powder and sintered as above, to produce a steel sea water valve with a thick titanium coating which is impervious to sea water and thus corrosion resistant. The current practice is to cast the titanium valve entirely from titanium at considerable cost. Adhesion of the surface coat prior to sintering can be improved by using small amounts of a residue binder in the solvent such as agar, polysaccharides or any of the binders used in conventional metal injection mold (MIM) processing. Thus the invention may be applied to coat parts which are subject to aggressive environments.

[0067] Another aspect of the invention, as discussed in FIG. 3, is to coat the inside of a mold with the fine powder, then fill it with coarse metal preform powder, e.g. 500 micron, and sinter.

[0068] A slurry was prepared by suspending 165 g of less than 325 mesh, or less than 44 micron Cp Ti (commercially pure titanium) in a solution consisting of 200 ml of distilled water, 600 g of poly vinyl alcohol, 2 ml Nalco 8815 wetting agent and 0.2 ml Nalco H10 antifoaming agent. The slurry was then poured into a conventional porous, alumina shell crucible 60, FIG. 9. The crucible was mildly shaken to distribute the slurry and coat the mold surface. The excess slurry was drained from the mold leaving the dark titanium powder coating 62. The crucible was dried at 100°C., filled with less than 35 mesh, or less than 500 micron, powder and sintered for 120 minutes at 1500°C. in a vacuum furnace. The resulting near net shape may be finish machined or compacted as above.

[0069] While only preforms of Ti-6Al-4V have been described, this process can be used with any powder. Stainless steels, tool steels, and super alloys are just a few examples that are sintered or hot isostatically pressed to make parts. Although PREP powders and gas atomized powders are preferred for the preform due to their spherical shape, any type of powder such as water atomized or chemically reduced powder will also work. Although spherical gas atomized powder was employed for the coatings, non-spherical powder such as is made by the hydride dehydride process or by comminution will offer an advantage due to their blocky shape which will help them interlock with the surface pores of the preform.

[0070] While the above work has used powder less than 44 microns in diameter, this is not a necessary limitation of the invention as there are many factors that could make smaller or larger sizes more desirable. The required powder size will depend on the powder's sintering kinetics for a specific alloy. Some alloys may require finer powder, while other alloys may allow the use of coarser powder. Finer powder will provide more rapid sintering and better adhesion of the unsintered coat, while coarser powder will be less expensive and result in less oxygen contamination.

[0071] Some typical powders and their respective sinter and melting points are shown in the following table. However, this in no way limits the type and sizes of metal powders which may be used, or the sintering temperature for which the method of the present invention will produce a gas impermeable, closed porosity coating. Indeed, the method of the present invention may also be accomplished with powders and preforms that produce a eutectic. That is, a powder with a high melting point, such as iron (1538°C.), may be applied to a preform with a similarly high melting point such as titanium (1668°C). The two metals together will actually melt below the melting point of either metal, approximately 1085°C., to form a eutectic. Particle Size (range) Sinter (range) Melting Point Metal Powder (μ) (° C.) (° C.) Al 2-5 1200  660 Ti <44 1120-1415 1600 1080 steel <37 1043-1415 1490 Stainless steel <37  962-1306 1375 316 Cp Ti <44 1120-1520 1600 PWA 771 <44  931-1264 1330 F-75  <100 1015-1377 1450

[0072] Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention.

[0073] Other embodiments will occur to those skilled in the art and are within the following claims: 

What is claimed is:
 1. A method of making a low density part with a closed porosity surface coating comprising: applying, to a porous powder preform, a coating of powder finer than that of the preform which is sinterable to a near full density below the melting temperature of the powder; and heating the coated preform to sinter the coating to form a near full density, gas impermeable, closed porosity surface coating on the preform.
 2. The method of claim 1 further including applying an increased pressure to the closed porosity coating for transmitting the force of the pressure through the coating to consolidate the porous powder preform to a predetermined density.
 3. The method of claim 2 in which the increased pressure includes gas pressure.
 4. The method of claim 2 in which the preform is consolidated to near full density.
 5. The method of claim 1 in which the step of applying includes adding the metal powder to a carrier and applying the carrier to the preform.
 6. The method of claim 5 in which the carrier includes a binder for aiding the powder in adhering to the preform.
 7. The method of claim 1 in which applying includes brushing the powder onto the preform.
 8. The method of claim 1 in which applying includes dipping the preform into the powder.
 9. The method of claim 1 in which applying includes spraying the powder onto the preform.
 10. The method of claim 9 in which the powder is melted and sprayed on as molten droplets.
 11. The method of claim 1 further including placing the powder coated preform in a vacuum furnace and reducing the pressure to remove any gas from the porous preform prior to heating.
 12. The method of claim 1 in which the powder is spherical.
 13. The method of claim 1 in which the coating of powder is the same material as the preform.
 14. A method of making a high density part from a low density part comprising: applying, to a porous powder preform, a coating of powder finer than that of the preform which is sinterable to a near full density below the melting temperature of the powder coating; heating the powder coated preform to sinter the coating to form a near full density, gas impermeable, closed porosity surface coating on the preform; and applying increased pressure to the closed porosity coating for transmitting the force of the pressure through the coating to consolidate the porous powder preform to a predetermined density.
 15. The method of claim 14 in which the increased pressure includes gas pressure.
 16. The method of claim 14 in which the predetermined density is near full density.
 17. The method of claim 14 in which the step of applying includes adding the metal powder to a carrier and applying the carrier to the preform.
 18. The method of claim 17 in which the carrier includes a binder for aiding the powder in adhering to the preform.
 19. The method of claim 14 in which applying includes brushing the powder onto the preform.
 20. The method of claim 14 in which applying includes dipping the preform into the powder.
 21. The method of claim 14 in which applying includes spraying the powder onto the preform.
 22. The method of claim 21 in which the powder is melted and sprayed on as molten droplets.
 23. The method of claim 14 further including placing the coated preform in a vacuum furnace and reducing the pressure to remove any gas from the porous preform prior to heating.
 24. The method of claim 14 in which the powder is spherical.
 25. The method of claim 14 in which the coating of powder is the same material as the preform.
 26. A method of making a metallic part with a closed porosity surface coating comprising: applying, to a metallic part, a coating of powder sinterable to a near full density below the melting temperature of the powder; and heating the coated part to sinter the powder coating to form a near full density, gas impermeable, closed porosity surface coating.
 27. The method of claim 26 further including applying an increased pressure to the closed porosity surface coating for transmitting the force of the pressure through the coating to consolidate the porous powder preform to a predetermined density.
 28. The method of claim 27 in which the increased pressure includes gas pressure.
 29. The method of claim 27 in which the preform is consolidated to near full density.
 30. The method of claim 26 in which the step of applying includes adding the metal powder to a carrier and applying the carrier to the preform.
 31. The method of claim 30 in which the carrier includes a binder for aiding the powder in adhering to the preform.
 32. The method of claim 26 in which applying includes brushing the powder onto the preform.
 33. The method of claim 26 in which applying includes dipping the preform into the powder.
 34. The method of claim 26 in which applying includes spraying the powder onto the preform.
 35. The method of claim 33 in which the powder is melted and sprayed on as molten droplets.
 36. The method of claim 26 further including placing the coated preform in a vacuum furnace and reducing the pressure to remove any gas from the porous preform prior to heating.
 37. The method of claim 26 in which the powder is spherical.
 38. The method of claim 26 in which the coating of powder is the same material as the preform.
 39. A low density powder metal near net shape part with a closed porosity coating produced by: applying, to a porous powder preform, a coating of powder finer than that of the preform which is sinterable to a near full density below the melting temperature of the powder; and heating the coated preform to sinter the coating to form a near full density, gas impermeable, closed porosity surface coating on the preform.
 40. A near net shape part comprising: a porous metal powder preform; and a fine metal powder coating disposed on said preform, the metal powder coating sintered below the melting temperature of the powder coating to form a full density, gas impermeable closed porosity coating on the preform.
 41. The near net shape part of claim 40 in which the metal preform had a near full density.
 42. A method of making a near net shape full density coating comprising: applying a powder coating to a powder metal preform, the powder coating sinterable to full density; and heating the coated preform to sinter the coating to form a closed porosity, gas impermeable near net shape coating.
 43. The method of claim 42 in which the powder of the coating is finer than that of the preform.
 44. A method of making a low density part with a gas impermeable coating comprising: applying, to a porous powder preform, a coating of powder finer than that of the preform and having a lower melting point than the preform; and heating the powder coated preform to melt the coating such that the coating reacts with the preform to form a near full density, gas impermeable, closed porosity surface coating on the preform. 